Patent GB-1431378-A (GB)
Title: Colour information translating systems
IPC: H04N9/04
Filed: 1972-04-19
Abstract: 1431378 Colour television RCA CORPORATION 18 April 1973 [19 April 1972] 18036/72 Heading H4F Received primary colour signals from a film scanner, or a received NTSC colour television signal are processed using comb filter apparatus to provide a composite signal derived by adding an NTSC chrominance signal at, e.g. 1À53 MHz, which, with the use of comb filter apparatus, has nulls at multiples of the line frequency, to a luminance signal, the low frequency portion of which has not been comb-filtered, but the upper frequency portion of which, which overlaps with the chrominance signal, has been comb-filtered to have nulls at odd multiples of half the line frequency. The resulting composite signals may be recorded, the played back signal being converted into NTSC form using comb filter apparatus to separate the luminance and chrominance signals. The composite signal may alternatively be formed with a PAL chrominance signal, in which case quarter-line offset rather than half-line offset may be used. In one embodiment, Fig. 1a, receiving an NTSC colour television signal, the low frequency portion of the received signal is removed by a high pass filter 20, comb filter apparatus 30 providing an output T 3  a comb-filtered chrominance signal and at output T 4  a luminance signal, the low frequency portion of which, derived via trimming delay 32, is not combfiltered, and the upper frequency portion of which is comb-filtered. The chrominance signal at T 3  is frequency-shifted to 1À53 MHz by modulator 43 and band-pass filter 47, the composite signal being derived from adder 50. In another embodiment (Fig. 3, not shown) adder 35 is omitted, delay line 31 and subtractor 33 are connected to the NTSC input, the high pass filter (120) is connected to the output of subtractor 33, and the luminance signal is derived from a subtractor subtracting the output of the high pass filter (120) from the output of the one-line delay line 31.  In a further embodiment a further adder (40, Fig. 2a, not shown) is included between adder 35 and delay unit 42 to add the output of low pass filter 21 to the luminance signal to compensate for the amplitude increase of the luminance signal in the band passed by high pass filter 20 due to the action of adder 35. In embodiments receiving primary colour signals from a film scanner (200, Figs. 8, 9, not shown) luminance and NTSC chrominance (at 1À53 MHz) signals are derived and separately comb-filtered, the luminance signal being high pass (or band-pass) filtered before or after comb-filtering, the output of a low pass (or band elimination) filter connected to receive the input (or one-line delayed) luminance signal, being added to the resulting comb-filtered luminance and chrominance signals. In apparatus for deriving an NTSC signal from a recorded composite signal derived as above, the composite signal is separated into luminance and chrominance signals using comb-filter apparatus 70, Fig. 1b, employing two one-line delays 71, 72 and high pass filter 77, the output of T c  being a luminance signal with the low frequency portion, received via trimming delay 75, not comb-filtered, and the upper frequency portion comb-filtered, and the output at Tb being a comb-filtered chrominance signal which is frequency shifted by modulator 83 and band-pass filter 87 to be of NTSC subcarrier frequency. To compensate for the amplitude increase of the luminance signal in the band passed by high pass filter 77 due to the action of adder 76, a further adder (101, Fig. 2b, not shown) may be included between adder 76 and delay unit 82, which further adder receives the output of a low pass filter connected to the output of delay line 71. Vertical aperture correction may be carried out by coupling to the further adder (101) the output of an inverter (102) connected to the output of low pass filter 77A. The high pass filter 77 may be replaced by a band-pass filter (150, Figs. 4, 5, not shown) connected to the input of comb-filter apparatus 70, the output of a band elimination filter (150A) forming part of the band pass filter, being added to the comb-filtered luminance signal from comb-filtered apparatus 70. The modulator 83 may be replaced by a modulator at the input to comb-filter apparatus 70 (Figs. 5-7, not shown) and the frequency of reference oscillations fed to this modulator may be controlled in response to the detected frequency of the colour bursts in the chrominance signal input to adder 90, to reduce jitter. The output of this modulator may be selected to be of NTSC subcarrier frequency in which case, with comb-filter apparatus similar to that of Fig. 1b but omitting filter 77, the resulting comb-filtered luminance signal is applied to a modulator (156, Fig. 5, not shown) to shift its frequency back to that of the equivalent received signal. Alternatively the luminance signal may be derived by subtracting the comb-filtered chrominance signal from the output of delay line 71 (Fig. 7, not shown) and envelope detecting and filtering the result. This method may also be used if comb filter apparatus 70 separates from the modulator output a frequency equal to 1À53 MHz + 2X NTSC subcarrier frequency, in which case an additional modulator (156<SP>1</SP>, Fig. 6, not shown) is required to derive the NTSC frequency chrominance signal.